The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display device with a desirable display quality as viewed from an inclined direction.
In the prior art, liquid crystal display devices using nematic liquid crystal materials have been widely used as segment-type numeric display device in timepieces, calculators, etc. In recent years, they have been widely used in word processors, notebook personal computers, and car navigation systems. The variety of applications of such liquid crystal display devices is still expanding, and they are now used in direct-viewing type large-screen TVs having a diagonal dimension of 20 to 30 inches.
A liquid crystal display device of this type typically includes a pair of substrates opposing each other with a liquid crystal layer interposed therebetween, and electrodes, wired lines, etc., provided on the substrates for turning ON/OFF pixels. An active matrix liquid crystal display device, for example, includes pixel electrodes which are arranged in a matrix pattern for applying voltages across the liquid crystal layer, and active elements such as field effect transistors which are provided on one of the substrates, along with the electrodes and the wired lines, as switching means for selectively applying different potentials to the respective pixel electrodes. Moreover, a liquid crystal display device capable of color display includes a color filter layer provided on one of the substrates. The color filter layer includes color filter portions of different colors such as red, green and blue. The display modes known in the art for use with such a liquid crystal display device can be classified as follows based on the twist angle of the nematic liquid crystal molecules:
(1) Twist nematic liquid crystal display mode (hereinafter, referred to as the “TN mode”) in which the twist angle of the nematic liquid crystal molecules is 90° between the pair of substrates; and
(2) Super twisted nematic liquid crystal display mode (hereinafter, referred to as the “STN mode”) in which the twist angle of the nematic liquid crystal molecules is greater than 90° between the pair of substrates.
liquid crystal display devices of these display modes have a viewing angle dependence due to which a change in the contrast ratio of the displayed image or an inversion phenomenon occurs depending upon the direction (azimuthal angle: direction within the display plane) or angle (vertical angle: direction with respect to the normal to the display plane) from which the viewer observes the display plane (the term “viewing angle dependence” as used herein includes both the azimuthal angle dependence and the vertical angle dependence). This prevents one from obtaining a wide viewing angle characteristic. For example, in the 6 o'clock direction of the display plane (the downward direction defined in the display plane seen as a clockface), the gray level inversion phenomenon may occur even when the vertical angle is slightly inclined. In the 12 o'clock direction (the upward direction), a sufficient contrast ratio may not be obtained, thereby rendering the overall displayed image whitish. Also in the 3 o'clock and 9 o'clock directions (the horizontal directions), the gray level inversion phenomenon may occur, thereby deteriorating the display quality.
In an attempt to solve this problem, Japanese Patent No. 2866540 and Japanese Laid-Open Patent Publication No. 9-120005, for example, disclose a liquid crystal display device employing the combination of a liquid crystal cell of the TN mode as described above and a phase compensation element whose index ellipsoid is inclined. In the disclosed liquid crystal display device, the liquid crystal cell and the phase compensation element are arranged so that the pretilt direction of the liquid crystal molecules in each pixel in the vicinity of an alignment film is opposite to the inclination direction of the principal axis of the index ellipsoid of the phase compensation element. Therefore, the positive uniaxial refractive index anisotropy of the liquid crystal molecules in each pixel which are raised by the applied voltage is compensated for by the negative uniaxial refractive index anisotropy of the phase compensation element. Thus, the refractive index anisotropy of the liquid crystal molecules in the vicinity of the substrate interface which are not raised upon application of a voltage is also effectively optically compensated for. As a result, the gray level inversion phenomenon in the 6 o'clock direction (downward direction) is suppressed, and the contrast ratio in the 12 o'clock direction (upward direction) is also improved, thereby widening the vertical angle in these directions. Moreover, the inversion phenomenon no longer occurs also in the 3 o'clock and 9 o'clock directions (the horizontal directions), thereby widening the vertical angle in these directions. As described above, by using a phase compensation element whose index ellipsoid is inclined, it is possible to widen the vertical angle in the vertical directions (the upward and downward directions) and also in the horizontal directions (the leftward and rightward directions) as viewed from the viewer.
The viewing angle dependence as described above is observed in a TN mode liquid crystal display device in which the pretilt direction of the liquid crystal molecules near the center of the liquid crystal layer along the thickness thereof (so called the “normal viewing direction”) is set to be the downward direction (6 o'clock direction). In a typical TN mode liquid crystal display device, the normal viewing direction is set as described above so that the maximum contrast ratio can be obtained when the vertical angle is inclined in the downward direction (the 6 o'clock direction) with respect to the normal to the display plane. It is assumed that the liquid crystal layer of a TN mode liquid crystal display device illustrated herein has such an arrangement unless otherwise noted. That is, the vertical directions include the normal viewing direction and the horizontal directions are perpendicular to the normal viewing direction.
Currently, however, there is a demand for a liquid crystal display device having an even better viewing angle, display quality and color reproducibility, and a sufficient display quality to satisfy such a demand may not be realized by simply combining a TN mode liquid crystal cell with a phase compensation element whose index ellipsoid is inclined as described above.
The above-described conventional liquid crystal display device displays an image by changing the orientation of the liquid crystal layer through voltage application thereacross so as to change the transmittance for light from a light source which is incident upon the liquid crystal layer, thereby obtaining a black, white or intermediate brightness. As described above, the conventional liquid crystal display device has a vertical angle dependence because the apparent retardation value of the liquid crystal layer varies depending upon the vertical angle, and the vertical angle dependence is compensated for by the phase compensation element.
However, the respective retardations of the liquid crystal layer and the phase compensation element each have a wavelength dispersion (wavelength dependence), and the respective wavelength dispersions typically differ from each other. Therefore, in the above-described conventional liquid crystal display device, even if the retardation value of the liquid crystal layer and the retardation value of the phase compensation element are optimized in the normal direction (the direction normal to the display plane) for a white display and an intermediate gray level display, coloring occurs when viewed from an inclined direction (a direction inclined from the normal to the display plane) due to the difference in wavelength dispersion of retardation between the liquid crystal layer and the phase compensation element. Particularly, when the vertical angle is inclined in the 3 o'clock or 9 o'clock direction (horizontal direction), the display image becomes yellowish.